The present invention relates to a silicone grease composition or, more particularly, to a silicone grease composition having greatly improved lubrication performance relative to the extreme pressure lubrication between steel and steel, which is the most serious problem in conventional silicone grease compositions to be used for lubrication bearings of various machines and the like.
It is well known that silicones, i.e. organopolysiloxane products, in general have excellent properties such as the very small temperature dependency of viscosity, shearing characteristics, chemical stability against heat and oxidation and so on as compared with ordinary organic materials. Silicone grease compositions prepared from a silicone fluid as the base oil are also excellent in these regards. One of the serious problems in the silicone grease compositions is that, in comparison to most of mineral oil-based and synthetic lubricating oil-based grease compositions, silicone grease compositions are generally poor in the performance of boundary lubrication, especially, between steel and steel so that satisfactory lubrication is not obtained under conditions of extereme pressure at high velocity and under high load, which greatly limits the applicability of silicone greases in the lubrication of most of heavy-duty machines.
In this connection, various proposals and attempts have been hitherto made to improve the lubricating performance of silicone greases, mainly, by admixing the silicone grease with various kinds of special additives. The additives proposed so far include fatty acids and derivatives thereof as an oiliness improver and organic compounds of halogen, phosphorus and sulfur as an extreme pressure additive to improve boundary lubrication. It is known that a great improvement can be obtained in the capacity of a silicone grease to withstand a heavy load by admixing the silicone grease with a salt of a dialkyl dithiocarbamic acid with a metal or metalloid element such as zinc, lead, antimony and the like as an extreme pressure additive. Additives of this type are not fully satisfactory in silicone grease compositions since the consistency of the silicone grease admixed with such an additive is increased within a relatively short time when the grease is exposed of time an they or used at high temperatures over a period to eventually lose their grease-like consistency and become resinous.
U.S. Pat. No. 3,642,646 teaches addition of an antimony salt to a silicone grease prepared from a fluorine-containing organopolysiloxane fluid as the base oil. Although a great improvement can be obtained by this means in the capacity of the grease to withstand heavy loads, such a silicone grease composition is defective due to the decrease in the thermal stability thereof at a high temperature of, for example, 250.degree. C. or higher.